Optical transmitter

ABSTRACT

The present invention is intended to prevent a light-emitting diode from emitting light continuously in the case when the level at an input terminal is fixed high because of software or the like and to avoid various problems, such as battery exhaustion and breakdown of the light-emitting diode, in PDAs, cellular phones, etc. For these purposes, a high-pass filter  21  for passing the high-frequency components of an optical transmission input signal having a pulse waveform and a binary circuit  22  for binarizing the output signal of the high-pass filter  21  so as to be returned to a pulse waveform are provided in the preceding stage of a light-emitting device driving circuit  23  for driving a light-emitting diode  8  for optical transmission.

TECHNICAL FIELD

Infrared communication systems are widely used for informationcommunication apparatuses, such as notebook personal computers, cellularphones, PDAs (Personal Digital Assistants).

Such an infrared communication system comprises a lightreceiving/emitting module and a system LSI for electrically processingdata. The light receiving/emitting module has an optical transmitter foroptically transmitting data and an optical receiver for opticallyreceiving data in one package.

Although various standards have been proposed for the infraredcommunication systems, an infrared communication system wherein the IrDAstandard is adopted is mainstream in these years.

The present invention relates to an optical transmitter for use in theconfiguration of the above-mentioned infrared communication system.

BACKGROUND TECHNOLOGY

The above-mentioned conventional optical transmitter has a circuitconfiguration shown in FIG. 7. In other words, as shown in FIG. 7, aninput terminal 51 is connected to one terminal of a resistor 52, and theother terminal of the resistor 52 is connected to the base of an NPNbipolar transistor 53 for driving a light emitting device. The collectorof the NPN bipolar transistor 53 is connected to the cathode of alight-emitting diode 54 for optical transmission, and the emitter of theNPN bipolar transistor 53 is connected to one terminal of a resistor 55.The anode of the light-emitting diode 54 is connected to a power supplyterminal 56, and the other terminal of the resistor 55 is connected to aground terminal 57.

In the above-mentioned optical transmitter, when an optical transmissioninput signal applied to the input terminal 51 becomes high, the NPNbipolar transistor 53 conducts, a current flows from the power supplyterminal 56 to the light-emitting diode 54, and the light-emitting diode54 emits light. In addition, when the optical transmission input signalapplied to the input terminal 51 becomes low, the NPN bipolar transistor53 shuts off, no current flows to the light-emitting diode 54, and thelight-emitting diode 54 stops light emission.

In the case of this circuit configuration, when an optical transmissioninput signal having an amplitude VIN shown in the waveform (a) of FIG.8, that is, the output signal from a system LSI, is input to the inputterminal 51 of the optical transmitter, the optical output from thelight-emitting diode 54 has the same waveform as that of the outputsignal from the system LSI as shown in the waveform (b) of FIG. 8.

However, in the case when the potential of the input terminal 51, thatis, the optical transmission input signal, is fixed high as shown in thewaveform (a) of FIG. 9 because of the software or the like of the systemLSI, the NPN bipolar transistor 53 maintains conducting. As a result,the light-emitting diode 54 emits light continuously as shown in thewaveform (b) of FIG. 9.

Hence, it has been pointed out that various problems, such as batteryexhaustion and breakdown of the light-emitting diode 54, may occur inPDAs and cellular phones.

For the prevention of the above-mentioned problems, a countermeasure istaken by disposing a protection circuit in the preceding stage of theinput terminal 51 of the optical transmitter in many cases.

This protection circuit is configured to measure the pulse width of theoptical transmission input signal by using a timer or the like and toforcibly stop the optical transmission input signal when the pulse widthexceeds a certain time width.

In the conventional circuit configuration shown in FIG. 7, as describedabove, in the case when the potential of the input terminal 51, that is,the optical transmission input signal, is fixed high because of thesoftware or the like, the light-emitting diode 54 emits lightcontinuously. As a result, various problems, such as battery exhaustionand breakdown of the light-emitting diode 54, are caused in PDAs andcellular phones.

Furthermore, the configuration wherein the protection circuit isdisposed in the preceding stage of the input terminal 51 of the opticaltransmitter is complicated, thereby causing a problem of high cost.

DISCLOSURE OF INVENTION

The present invention is intended to provide an optical transmittercapable of automatically stopping the light-emitting operation of alight-emitting device when the pulse width of an optical transmissioninput signal exceeds a predetermined value owing to a malfunction, suchas the case when the optical transmission input signal is fixed high,capable of preventing battery exhaustion and breakdown of thelight-emitting device due to the malfunction, and capable of attaining aconfiguration for the purposes simply at low cost.

The optical transmitter in accordance with the present inventioncomprises a high-pass filter for passing the high-frequency componentsof an optical transmission input signal having a pulse waveform, abinary circuit for binarizing the output signal of the high-pass filterso as to be returned to a pulse waveform, a light-emitting device foroptical transmission, and a light-emitting device driving circuit fordriving the light-emitting device depending on the output signal of thebinary circuit.

With this configuration, the optical transmission input signal having apulse waveform is not directly input to the light-emitting devicedriving circuit; instead, the signal is once passed through thehigh-pass filter so that the pulse waveform is differentiated, thedifferentiated signal is binarized by the binary circuit so as to bereturned to a pulse waveform, and then the signal is input to thelight-emitting device driving circuit. Hence, the optical transmissioninput signal having a pulse width shorter than the predetermined timewidth determined by the time constant of the high-pass filter and thebinarization threshold value of the binary circuit is output from thebinary circuit while its pulse width is unchanged.

However, with respect to the optical transmission input signal having apulse width longer than the above-mentioned predetermined time width,when the signal is passed through the high-pass filter, its level lowersgradually; when the predetermined time passes, the level of the outputsignal of the high-pass filter becomes lower than the binarizationthreshold value of the binary circuit. As a result, even if the opticaltransmission input signal having a pulse width longer than theabove-mentioned predetermined time width is input, a signal having apulse width longer than the predetermined time width is not output fromthe binary circuit.

Hence, in the case when the optical transmission input signal is fixedhigh because of software or the like, or in the case when the pulsewidth of the optical transmission input signal becomes longer than apulse width assumed in an applicable communication system, thelight-emitting diode emits light only during the period of time of thepredetermined pulse width but does not emit light during the period oftime longer than that.

Therefore, it is possible to prevent battery exhaustion of the batteryfor supplying electricity to the optical transmitter and to preventbreakdown of the light-emitting diode.

In addition, as the configuration for the above-mentioned purpose, thehigh-pass filter and the binary circuit should only be provided, and thecircuit constant of the high-pass filter should only be set properlydepending on the time width of the optical transmission signal to beshuts off, whereby the configuration is simple and can be attained atlow cost.

In the above-mentioned optical transmitter in accordance with thepresent invention, the high-pass filter comprises an L-type circuitincluding a capacitor and a resistor, for example.

With this configuration, since the high-pass filter comprises thecapacitor and the resistor, the configuration is simple and can beattained at low cost.

In the above-mentioned optical transmitter in accordance with thepresent invention, the binary circuit comprises two-stage invertersconnected in series, for example.

With this configuration, since the binary circuit comprises thetwo-stage inverters connected in series, the configuration is simple andcan be attained at low cost.

In the above-mentioned optical transmitter in accordance with thepresent invention, the inverters are formed of CMOS inverters, forexample.

With this configuration, since the inverters are formed of CMOSinverters, no standby current is required, and the power consumption canbe made lower than that of the configuration wherein the inverters areformed of bipolar transistors.

In the above-mentioned optical transmitter in accordance with thepresent invention, the light-emitting device driving circuit comprises abipolar transistor, for example. The output signal of the binary circuitis input to the base of the bipolar transistor, and the bipolartransistor interrupts the current supplied to the light-emitting devicedepending on the output signal of the binary circuit.

In the above-mentioned optical transmitter in accordance with thepresent invention, the light-emitting device driving circuit comprises aDarlington circuit including two-stage bipolar transistors connected inseries, for example. The output signal of the binary circuit is input tothe base of the first-stage bipolar transistor of the Darlingtoncircuit, and the next-stage bipolar transistor of the Darlington circuitinterrupts the current supplied to the light-emitting device dependingon the output signal of the binary circuit.

With this configuration, the output signal of the binary circuit isapplied to the base of the first-stage bipolar transistor, instead ofthe base of the next-stage bipolar transistor that directly drives thelight-emitting diode, whereby, as the binary circuit, a circuit havinglow current drive capability can be used.

Furthermore, the output signal of the binary circuit is applied betweenthe two base-emitter junctions, being connected in series, of thefirst-stage and next-stage bipolar transistors constituting theDarlington circuit. In other words, the output signal is applied betweenthe base of the first-stage bipolar transistor and the emitter of thenext-stage bipolar transistor. As a result, when noise is generated atthe output of the binary circuit, if the level of the noise is nothigher than that in the case when the light-emitting device drivingcircuit comprises a one-stage bipolar transistor, the light-emittingdevice does not emit light. Hence, noise immunity can be improved.

In the above-mentioned optical transmitter in accordance with thepresent invention, the light-emitting device driving circuit comprises aMOS transistor, for example. The output signal of the binary circuit isinput to the gate of the MOS transistor, and the MOS transistorinterrupts the current supplied to the light-emitting device dependingon the output signal of the binary circuit.

With this configuration, since the drain-source voltage of the MOStransistor, which determines the lower limit of the operating voltage,is lower than the collector-emitter voltage of the bipolar transistor,low-voltage operation is possible.

In the above-mentioned optical transmitter in accordance with thepresent invention, the light-emitting device is formed of alight-emitting diode, for example.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing the configuration of an opticaltransmitter in accordance with a first embodiment of the presentinvention;

FIG. 2 is a waveform diagram showing the operation of the opticaltransmitter in accordance with the first embodiment of the presentinvention;

FIG. 3 is a waveform diagram showing the operation of the opticaltransmitter in accordance with the first embodiment of the presentinvention;

FIG. 4 is a circuit diagram showing the configuration of an opticaltransmitter in accordance with a second embodiment of the presentinvention;

FIG. 5 is a circuit diagram showing the configuration of an opticaltransmitter in accordance with a third embodiment of the presentinvention;

FIG. 6 is a circuit diagram showing the configuration of an opticaltransmitter in accordance with a fourth embodiment of the presentinvention;

FIG. 7 is a circuit diagram showing the configuration of theconventional optical transmitter;

FIG. 8 is a waveform diagram showing the operation of the conventionaloptical transmitter; and

FIG. 9 is a waveform diagram showing the operation of the conventionaloptical transmitter.

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment: Corresponding to Claims 1, 2, 3 and 5)

FIG. 1 is a circuit diagram showing the configuration of an opticaltransmitter in accordance with a first embodiment of the presentinvention. As shown in FIG. 1, this optical transmitter comprises ahigh-pass filter 21 for passing the high-frequency components of anoptical transmission input signal having a rectangular pulse waveform, abinary circuit 22 for binarizing the output signal of the high-passfilter 21 so as to be returned to a pulse waveform, a light-emittingdiode 8 serving as a light-emitting device for optical transmission, anda light-emitting device driving circuit 23 for driving thelight-emitting diode 8 depending on the output signal of the binarycircuit 22.

The high-pass filter 21 comprises an L-type circuit including acapacitor 2 and a resistor 3, for example.

The binary circuit 22 comprises two-stage inverters 4 and 5 connected inseries, for example. The inverters 4 and 5 are each formed of a bipolartransistor, for example.

The light-emitting device driving circuit 23 comprises an NPN bipolartransistor 7 and resistors 6 and 7. The NPN bipolar transistor 7interrupts the current supplied to the light-emitting diode 8 dependingon the output signal of the binary circuit 22, which is input to itsbase. The resistor 6 is inserted between the base of the NPN bipolartransistor 7 and the binary circuit 22. The resistor 9 is insertedbetween the emitter of the NPN bipolar transistor 7 and a groundterminal 11.

The connection relationship among the devices of the optical transmitterwill be described below specifically. One terminal of the capacitor 2 isconnected to an input terminal 1, one terminal of the resistor 3 and theinput terminal of the inventor 4 are connected to the other terminal ofthe capacitor 2, and the input terminal of the inventor 5 is connectedto the output terminal of the inventor 4. One terminal of the resistor 6is connected to the output terminal of the inventor 5, and the otherterminal of the resistor 6 is connected to the base of the NPN bipolartransistor 7. The cathode of the light-emitting diode 8 is connected tothe collector of the NPN bipolar transistor 7, and the anode of thelight-emitting diode 8 is connected to a power supply terminal 10. Oneterminal of the resistor 9 is connected to the emitter of the NPNbipolar transistor 7, and the other terminal of the resistor 3 and theother terminal of the resistor 9 are connected to the ground terminal11.

For explanation of this embodiment, the node wherein the capacitor 2,the resistor 3 and the input terminal of the inventor 4 are connected isdesignated by numeral 31, and the node wherein the output terminal ofthe inventor 4 and the input terminal of the inventor 5 are connected isdesignated by numeral 32.

The waveform (a) of FIG. 2 shows the waveform at the input terminal 1 atthe time when a pulse waveform having a pulse width of 75 μsec, forexample, is input as the optical transmission signal to the inputterminal 1. Similarly, the waveform (b) of FIG. 2 shows the waveform atthe node 31. Similarly, the waveform (c) of FIG. 2 shows the waveform atthe node 32. Similarly, the waveform (d) of FIG. 2 shows the opticalpower of the light-emitting diode 8.

The waveform (a) of FIG. 3 shows the waveform at the input terminal 1 atthe time when a pulse waveform having a pulse width of more than 75μsec, for example, is input as the optical transmission signal.Similarly, the waveform (b) of FIG. 3 shows the waveform at the node 31.Similarly, the waveform (c) of FIG. 3 shows the waveform at the node 32.Similarly, the waveform (d) of FIG. 3 shows the optical power of thelight-emitting diode 8.

In FIGS. 2 and 3, V_(IN) is the amplitude of the optical transmissionsignal, and V_(T) is a threshold value at which the first-stage inventor4 of the binary circuit 22 distinguishes between high and low levels.

When the optical transmission input signal shown in the waveform (a) ofFIG. 2 is input to the input terminal 1, voltage V₃₁ shown in thewaveform (b) of FIG. 2 is output at the node 31. The opticaltransmission input signal shown in the waveform (a) of FIG. 2 is arectangular waveform signal having amplitude V_(IN), cycle T and duty3/16. In addition, the voltage V₃₁ shown in the waveform (b) of FIG. 2is represented by the following equation (1):V ₃₁ =V _(IN)*exp {−t/(C ₂ *R ₃)}  Equation (1)wherein C₂ designates the capacitance value of the capacitor 2, and R₃designates the resistance value of the resistor 3.

According to Equation (1), the voltage V₃₁ at the node 31 decreasesgradually at an attenuation rate of time constant C₂*R₃.

At this time, the capacitance value C₂ of the capacitor 2 and theresistance value R₃ of the resistor 3 are determined so that the voltageV₃₁ at the node 31 does not become lower than the threshold value VTuntil the input signal having a predetermined pulse width returns to thelow level.

For example, since the longest pulse in the IrDA standard is 75 μsec (inthe case of 2.4 kbps and duty 3/16), the capacitance value C₂ of thecapacitor 2 and the resistance value R₃ of the resistor 3 are determinedso as to satisfy the following equation (2):t=C ₂ *R ₃*ln(V _(IN) /V _(T))≧−75μsec  Equation (2)

In the case when it is assumed herein that the power supply voltage VINis 3.0 V and the threshold value VT is 1.4 V, and when it is assumedthat, for example,

-   -   C₂=100 pF and R₃=1 MΩ        Equation (2) can be satisfied.

Hence, the waveform (c) of FIG. 2, the inversion of the opticaltransmission input signal, is output at the node 32. Therefore, thelight-emitting diode 8 emits light during the period of the same pulsewidth as that of the optical transmission input signal to be input, asshown in the waveform (d) of FIG. 2. When the pulse width of the opticaltransmission input signal is shorter than 75 μsec, the light-emittingdiode 8 emits light during the same time as that of the pulse width.

Next, when the optical transmission input signal having a pulse width of75 μsec or more is input as shown in the waveform (a) of FIG. 3 becauseof the malfunction of the preceding-stage circuit (system LSI), thevoltage V₃₁ at the node 31 becomes lower than the threshold value V_(T)at the time when 75 μsec has passed after the rising, as shown in thewaveform (b) of FIG. 3. Hence, the voltage at the node 32 returns to thehigh level at the time when 75 μsec has passed after the rising of theoptical transmission input signal, regardless of the pulse width of theoptical transmission input signal. Therefore, the light-emitting diode 8emits light only 75 μsec at the longest, no matter how the pulse widthof the optical transmission input signal is long.

As described above, the circuit system wherein the optical transmissioninput signal having a pulse width of 75 μsec or more is not transmittedto the light-emitting diode 8 is obtained.

With this configuration, the optical transmission input signal having apulse waveform is not directly input to the light-emitting devicedriving circuit 23; instead, the signal is once passed through thehigh-pass filter 21 so that the pulse waveform is differentiated, thedifferentiated signal is binarized by the binary circuit 22 so as to bereturned to a pulse waveform, and then the signal is input to thelight-emitting device driving circuit 23. Hence, the opticaltransmission input signal having a pulse width shorter than thepredetermined time width determined by the time constant of thehigh-pass filter 21 and the binarization threshold value of the binarycircuit 22 is output from the binary circuit 22 while its pulse width isunchanged.

However, with respect to the optical transmission input signal having apulse width longer than the above-mentioned predetermined time width,when the signal is passed through the high-pass filter 21, its levellowers gradually; when the predetermined time passes, the level of theoutput signal of the high-pass filter 21 becomes lower than thebinarization threshold value of the binary circuit 22. As a result, evenif the optical transmission input signal having a pulse width longerthan the above-mentioned predetermined time width is input, a signalhaving a pulse width longer than the predetermined time width is notoutput from the binary circuit 22.

Hence, in the case when the optical transmission input signal is fixedhigh because of software or the like, the light-emitting diode 8 emitslight only during the period of time of the predetermined pulse widthbut does not emit light during the period of time longer than that.

Therefore, various problems being caused when the level at the inputterminal is fixed high or when the pulse width becomes longer than anassumed pulse width because of software or the like can be solved; forexample, it is possible to prevent battery exhaustion of the battery forsupplying electricity to the optical transmitter and to preventbreakdown of the light-emitting diode 8.

In addition, as the configuration for the above-mentioned purpose, thehigh-pass filter 21 and the binary circuit 22 should only be provided,and the circuit constant of the high-pass filter 21 should only be setproperly depending on the time width of the optical transmission signalto be shuts off, whereby the configuration is simple and can be attainedat low cost.

Furthermore, since the high-pass filter 21 comprises the capacitor 2 andthe resistor 3, the configuration is simple and can be attained at lowcost.

Still further, since the binary circuit 22 comprises the two-stageinverters 4 and 5 connected in series, the configuration is simple andcan be attained at low cost.

(Second Embodiment: Corresponding to Claim 7)

FIG. 4 is a circuit diagram showing the configuration of an opticaltransmitter in accordance with a second embodiment of the presentinvention. As shown in FIG. 4, in this optical transmitter, theconfiguration of a light-emitting device driving circuit 24 differs fromthat of the light-emitting device driving circuit 23 shown in FIG. 1. Inother words, in this light-emitting device driving circuit 24, anN-channel MOS transistor 12 is used instead of the NPN bipolartransistor 7. The other configurations are similar to those of theoptical transmitter shown in FIG. 1.

In the circuit diagram of FIG. 1, the operating voltage of the opticaltransmitter is determined by the forward voltage of the light-emittingdiode 8, the terminal voltage of the resistor 9 and thecollector-emitter voltage of the NPN bipolar transistor 7.

Since the operating current of the light-emitting diode 8 is determinedby light emission power, it is assumed herein that the current isconstant. Hence, the forward voltage of the light-emitting diode 8 andthe terminal voltage of the resistor 9 become constant, whereby thelower limit of the operating voltage is determined by thecollector-emitter voltage of the NPN bipolar transistor 7. Generally,the collector-emitter voltage of the NPN bipolar transistor 7 is about200 mV at the time of saturation.

On the other hand, in the case of the circuit diagram of FIG. 4, thelower limit of the operating voltage is determined by the drain-sourcevoltage of the N-channel MOS transistor 12 at the time of operation.This value, 10 mV, is lower than the collector-emitter voltage of theNPN bipolar transistor 7. Hence, the circuit shown in FIG. 4 can operateon a voltage lower than that of the circuit shown in FIG. 1. The effectsother than that described above are similar to those of the firstembodiment.

As described above, with the configuration of this embodiment, since thedrain-source voltage of the N-channel MOS transistor 12, whichdetermines the lower limit of the operating voltage, is lower than thecollector-emitter voltage of the bipolar transistor, low-voltageoperation is possible.

(Third Embodiment: Corresponding to Claim 6)

FIG. 5 is a circuit diagram showing the configuration of an opticaltransmitter in accordance with a third embodiment of the presentinvention. As shown in FIG. 5, in this optical transmitter, theconfiguration of a light-emitting device driving circuit 25 differs fromthat of the light-emitting device driving circuit 23 shown in FIG. 1. Inother words, in this light-emitting device driving circuit 25, insteadof the one-stage NPN bipolar transistor 7, a Darlington circuit having atwo-stage configuration wherein NPN bipolar transistors 17 and 7 areconnected in series is used. In this light-emitting device drivingcircuit 25, depending on the output signal of the binary circuit 22,input to the base of the first-stage NPN bipolar transistor 17, thenext-stage NPN bipolar transistor 7 interrupts the current supplied tothe light-emitting diode 8.

More specifically, the base of the first-stage NPN bipolar transistor 17is connected to the other terminal of the resistor 6, and the collectorof the NPN bipolar transistor 17 is connected to the power supplyterminal 10, and the emitter of the NPN bipolar transistor 17 isconnected to the base of the NPN bipolar transistor 7. The otherconfigurations are similar to those of the optical transmitter shown inFIG. 1.

In the circuit configuration shown in FIG. 1, in the case when noise isgenerated at the output of the inventor 5 constituting the binarycircuit 22 of the preceding stage, if the level of the noise becomes 0.3V or more, the transistor 7 gradually starts operating, and thelight-emitting diode 8 gradually starts light emission.

However, in the circuit configuration shown in FIG. 5, even in the casewhen noise is generated at the output of the inverter 5, since the NPNbipolar transistor 17 is inserted in the preceding stage of the NPNbipolar transistor 7 in the form of the Darlington connection, the NPNbipolar transistors 17 and 7 do not gradually start operation unless thelevel of the noise becomes 1.0 V or more.

In the configuration of this embodiment, the output signal of the binarycircuit 22 is applied to the base of the first-stage NPN bipolartransistor 17, instead of the base of the next-stage NPN bipolartransistor 7 that directly drives the light-emitting diode 8. As aresult, inverters having low current drive capability can be used as theinverters 4 and 5 constituting the binary circuit 22.

Furthermore, the output signal of the binary circuit 22 is appliedbetween the two base-emitter junctions, being connected in series, ofthe first-stage and next-stage NPN bipolar transistors 17 and 7constituting the Darlington circuit. In other words, the output signalof the binary circuit 22 is applied between the base of the first-stageNPN bipolar transistor 17 and the emitter of the next-stage NPN bipolartransistor 7. As a result, when noise is generated at the output of thebinary circuit 22, if the level of the noise is not higher than that inthe case when the light-emitting device driving circuit 25 comprisesonly the one-stage NPN bipolar transistor 7, the light-emitting diode 8does not emit light. Hence, noise immunity can be improved.

The other effects are similar to those of the first embodiment.

(Fourth Embodiment: Corresponding to Claim 4)

FIG. 6 is a circuit diagram showing the configuration of an opticaltransmitter in accordance with a fourth embodiment of the presentinvention. In this optical transmitter, the configuration of a binarycircuit 26 differs from that shown in FIG. 1. In other words, CMOSinverters 40 and 50 are used instead of the inverters 4 and 5 formed ofbipolar transistors. Furthermore, CMOS inverters can also be applied tothe circuits shown in FIGS. 4 and 5.

The inverters 40 and 50 have a circuit configuration described below. Inother words, the gate of the P-channel MOS transistor 13 and the gate ofthe N-channel MOS transistor 15 are connected to the resistor 3 and thecapacitor 2. In addition, the drain of the P-channel MOS transistor 13and the drain of the N-channel MOS transistor 15 are connected to thegate of the P-channel MOS transistor 14 and the gate of the N-channelMOS transistor 16. Furthermore, the drain of the P-channel MOStransistor 14 and the drain of the N-channel MOS transistor 16 areconnected to the resistor 6. Moreover, the sources of the P-channel MOStransistors 13 and 14 are connected to the power supply terminal 10.Still further, the sources of the N-channel MOS transistors 15 and 16are connected to the ground terminal 11.

With this embodiment, the inverters 40 and 50 are formed of CMOSinverters, whereby no standby current is required. Hence, the powerconsumption can be made lower than that of the first embodiment whereinthe inverters 4 and 5 formed of bipolar transistors are used. The othereffects are similar to those of the first embodiment.

In the above-mentioned embodiments, an NPN bipolar transistor or anN-channel MOS transistor is used for the light-emitting device drivingcircuit; however, instead of these, a PNP bipolar transistor or aP-channel MOS transistor can also be used to form the light-emittingdevice driving circuit.

1. An optical transmitter comprising a high-pass filter for passing thehigh-frequency components of an optical transmission input signal havinga pulse waveform, a binary circuit for binarizing the output signal ofsaid high-pass filter so as to be returned to a pulse waveform, alight-emitting device for optical transmission, and a light-emittingdevice driving circuit for driving said light-emitting device dependingon the output signal of said binary circuit.
 2. An optical transmitterin accordance with claim 1, wherein said high-pass filter comprises anL-type circuit including a capacitor and a resistor.
 3. An opticaltransmitter in accordance with claim 1, wherein said binary circuitcomprises two-stage inverters connected in series.
 4. An opticaltransmitter in accordance with claim 3, wherein said inverters areformed of CMOS inverters.
 5. An optical transmitter in accordance withclaim 1, wherein said light-emitting device driving circuit comprises abipolar transistor, the output signal of said binary circuit is input tothe base of said bipolar transistor, and said bipolar transistorinterrupts the current supplied to said light-emitting device dependingon the output signal of said binary circuit.
 6. An optical transmitterin accordance with claim 1, wherein said light-emitting device drivingcircuit comprises a Darlington circuit comprising two-stage bipolartransistors connected in series, the output signal of said binarycircuit is input to the base of the first-stage bipolar transistor ofsaid Darlington circuit, and the next-stage bipolar transistor of saidDarlington circuit interrupts the current supplied to saidlight-emitting device depending on the output signal of said binarycircuit.
 7. An optical transmitter in accordance with claim 1, whereinsaid light-emitting device driving circuit comprises a MOS transistor,the output signal of said binary circuit is input to the gate of saidMOS transistor, and said MOS transistor interrupts the current suppliedto said light-emitting device depending on the output signal of saidbinary circuit.
 8. An optical transmitter in accordance with claim 1,wherein said light-emitting device is formed of a light-emitting diode.